Some of the known steel pipe quenching techniques are as follows:
1) Both sides dip quenching of steel pipes in which steel pipe rotation is added to multiple constraint including pipe ends is markedly effective in preventing quench distortion, and also improves cooling capacity. Therefore, this technique is suitable for heat treatment (Q-T) of seamless steel pipes and electric resistance welded steel pipes, in particular, heavy wall steel pipes (refer to Non Patent Literature 1).2) In a both sides and axial stream dip quenching method, a heated steel pipe is dipped in a water tank, and quenching is performed while applying a cooling water flow (axial stream) to both sides of the steel pipe along the direction of axis. This method is advantageous in that its cooling capacity is large, and the structure of the equipment is simple (refer to paragraph [0002] of Patent Literature 1).3) In rotary quenching equipment for steel pipes, in order to minimize the difference in cooling history in the circumferential direction of pipe, a steel pipe is dipped in water in a water tank while rotating the steel pipe, and water injected from nozzles in the water is sprayed to both sides of the steel pipe to perform quenching. This equipment is placed in a final heat treatment line for carbon steel pipes (refer to paragraphs [0002] to [0003] of Patent Literature 2).
On the other hand, as the thin-walled (wall thickness: less than 1 inch) steel pipe whose strength can be stably adjusted to the target strength by Q-T, a steel pipe is known which has a composition (hereinafter referred to as the “composition A1”) containing, in percent by mass, 0.15% to 0.50% of C, 0.1% to 1.0% of Si, 0.3% to 1.0% of Mn, 0.015% of less of P, 0.005% or less of S, 0.01% to 0.1% of Al, 0.01% or less of N, 0.1% to 1.7% of Cr, 0.40% to 1.1% of Mo, 0.01% to 0.12% of V, 0.01% to 0.08% of Nb, 0.0005% to 0.003% of B, and further optionally one or two or more of 1.0% or less of Cu, 1.0% or less of Ni, 0.03% or less of Ti, 2.0% or less of W, and 0.001% to 0.005% of Ca, the balance being Fe and incidental impurities (refer to Patent Literature 3).